1. Field of the Invention
The present invention relates to a communication device including a surface acoustic wave device suitable for small-sized communication devices such as cellular phones, and more particularly, to a communication device including surface acoustic wave devices having a balance-unbalance conversion function.
2. Description of the Related Art
In recent years, technology in the field of reduction of the size and weight of communication devices, such as cellular phones, has made remarkable progress. In order to achieve such technology, development of components having multiple functions is progressing, as well as reduction of the number and size of components. With such background, in recent years, there has been intense development of surface acoustic wave devices having a balance-unbalance conversion function, also known as “balun”, for an RF (Radio Frequency) section of communication devices such as cellular phones, and such acoustic wave devices are primarily being used for GMS cellular phones.
As a surface acoustic wave device having different input impedance and output impedance, and a balance-unbalance conversion function, a surface acoustic wave device having a configuration as shown in FIG. 2 is known, for example. The surface acoustic wave device shown in FIG. 2 includes a first surface acoustic filter device 108, a second surface acoustic wave filter device 119 designed so as to output signals having a phase difference of 180° as compared to the first surface acoustic filter device 108, a single-port surface acoustic wave resonator 112 and 116, disposed on an unshown piezoelectric substrate. Such a device is disclosed in Japanese Unexamined Patent Application Publication No. 10-117123 or Japanese Unexamined Patent Application Publication No. 2001-308672.
The first surface acoustic wave filter device 108 includes three comb-shaped electrodes (which is also referred to as “interdigital transducer”, and will be referred to as “IDT” hereafter) i.e., the IDTs 103, 104, and 105 in the drawing, wherein the IDT 104 and the IDT 105 are disposed on left and right sides of the IDT 103, respectively. Furthermore, reflectors 106 and 107 are arranged so as to surround these IDTs 104, 103, and 105. That is to say, the first surface acoustic filter device 108 is a longitudinally-coupled resonator type filter having a configuration including three IDTs.
The single-port surface acoustic resonator 112 includes reflectors 110 and 111 that are arranged so as to surround an IDT 109. The single-port surface acoustic resonator 116 includes reflectors 114 and 115 that are arranged so as to surround an IDT 113. The IDT 103 is serially connected to the IDT 109. Furthermore, each of IDTs 104 and 105 are serially connected to the IDT 113.
With the first surface acoustic filter device 108, which is a longitudinally coupled-resonator type filter, a predetermined number of electrode fingers are provided with narrower pitch (which will be refereed to as “narrow-pitch electrode fingers” hereafter) around regions of the IDT 103 and the IDT 104 adjacent to one another, and the region of the IDT 103 and the IDT 105 adjacent to one another (regions 117 and 118 shown in FIG. 2) than with the other regions, and furthermore, these IDTs are arranged to have intervals of approximately half of the wavelength of the surface acoustic waves propagating thereon, thereby suppressing loss due to components escaping as bulk waves. Note that FIG. 2 shows an example having a small number of electrode fingers so as to simplify the drawing.
The second surface acoustic wave filter device 119 is a 3-IDT longitudinally coupled-resonator filter device having the same configuration as that of the first surface acoustic wave filter 108, except that the IDT at the middle portion is facing in the opposite direction. The second surface acoustic wave filter device 119 is connected to a single-port surface acoustic wave resonator 120 having the same configuration as the single-port surface acoustic wave resonator 112, and a single-port surface acoustic wave resonator 121 having the same configuration as with the single-port surface acoustic wave resonator 116.
With the first and second surface acoustic wave filters 108 and 119, one pair of terminals 122 and 123 are electrically connected to the load in parallel, and the other pair of the terminals 124 and 125 are electrically connected to the load in series, whereby the terminals connected to the load in parallel define an unbalanced signal terminal, and the terminals connected to the load in series define balanced signal terminals.
With the above-described surface acoustic wave device having a balance-unbalance conversion function, the circuit between the unbalanced signal terminal and one balanced signal terminal ideally exhibits the transmission properties of the same amplitude property and the inverse phase as compared with the circuit between the unbalanced signal terminal and the other balanced signal terminal. Note that actual differences in the amplitude and the phase between the aforementioned two circuits will be respectively referred to as “amplitude difference” and “phase difference” hereafter.
Specifically, the amplitude difference and the phase difference are obtained as follows. Assuming the surface acoustic wave device defining the aforementioned filter having a balance-unbalance conversion function is a 3-port device which includes a port 1 defining an unbalanced input terminal, and a port 2 and a port 3, defining balanced output terminals. In this case, the amplitude difference is defined as |A|, wherein A=|20 log(S21)|−|20 log(S31)|. On the other hand, the phase difference is defined as |B−180|, wherein B=|∠s21−∠s31|. The ideal amplitude difference and phase difference are 0 dB and 0°, respectively.
However, with the above-described conventional technique, the surface acoustic wave device having the configuration shown in FIG. 2 exhibits deviations of the amplitude difference and the phase difference from the ideal values, leading to a non-negligible problem in actual use. The reason is that while the electrode fingers of the IDT 103 which are adjacent to the IDT 104 and the IDT 105 are grounded, the electrode fingers of the IDT 127 which are adjacent to the IDT 126 and the IDT 128 define signal electrodes.
Specifically, with an IDT-coupling configuration wherein one of the coupled electrodes of the IDTs is grounded, and the other defines a signal electrode, the IDT-coupling configuration exhibits higher conversion efficiency of current in a resonant mode having an amplitude peak, than with an IDT-coupling configuration wherein both the coupled electrodes of the IDTs are grounded or serve as signal electrodes, leading to a problem that the former IDT-coupling configuration exhibits smaller insertion loss than with the latter IDT-coupling configuration within the pass band, particularly on the high-frequency side, and exhibits a wider pass band than therewith, resulting in a problem of deviation of phase therebetween.
With the conventional surface acoustic wave device having a balance-unbalance conversion function which includes the first surface acoustic wave filter 108 and the second surface acoustic wave filter 119, the aforementioned difference leads to deviation of the amplitude difference and the phase difference from the ideal values, i.e., 0 dB and 0°, respectively.